Android is designed around the unique requirements of mobile applications. In particular, Android recognizes that resources (memory and battery, for example) are limited on most mobile devices, and provides mechanisms to conserve those resources. The mechanisms are evident in the Android Activity Lifecycle, which defines the states or events that an activity goes through from the time it is created until it finishes running. The lifecycle is shown diagrammatically in Figure 1-1.
Your activity monitors and reacts to these events by instantiating methods that override the Activity class methods for each event:
Called when your activity is first created. This is the place you normally create your views, open any persistent datafiles your activity needs to use, and in general initialize your activity. When calling onCreate, the Android framework is passed a Bundle object that contains any activity state saved from when the activity ran before.
Called just before your activity becomes visible on the screen. Once onStart completes, if your activity can become the foreground activity on the screen, control will transfer to onResume. If the activity cannot become the foreground activity for some reason, control transfers to the onStop method. onResume
Called right after onStart if your activity is the foreground activity on the screen. At this point your activity is running and interacting with the user. You are receiving keyboard and touch inputs, and the screen is displaying your user interface. onResume is also called if your activity loses the foreground to another activity, and that activity eventually exits, popping your activity back to the foreground. This is where your activity would start (or resume) doing things that are needed to update the user interface (receiving location updates or running an animation, for example).
Called when Android is just about to resume a different activity, giving that activity the foreground. At this point your activity will no longer have access to the screen, so you should stop doing things that consume battery and CPU cycles unnecessarily. If you are running an animation, no one is going to be able to see it, so you might as well suspend it until you get the screen back. Your activity needs to take advantage of this method to store any state that you will need in case your activity gains the foreground again—and it is not guaranteed that your activity will resume. If the mobile device you are running on runs out of memory, there is no virtual memory on disk to use for expansion, so your activity may have to make way for a system process that needs memory. Once you exit this method, Android may kill your activity at any time without returning control to you. onStop
Called when your activity is no longer visible, either because another activity has taken the foreground or because your activity is being destroyed.
The last chance for your activity to do any processing before it is destroyed. Normally you'd get to this point because the activity is done and the framework called its finish method. But as mentioned earlier, the method might be called because Android has decided it needs the resources your activity is consuming.
It is important to take advantage of these methods to provide the best user experience possible. This is the first place in this book we've discussed how programming for mobile devices is different from programming for desktop devices, and there will be many more such places as you go through later chapters. Your users will appreciate it if you write your activities with the activity lifecycle in mind, and you will ultimately benefit.
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